US3605900A - Method of controlling fire - Google Patents

Abstract

A METHOD OF CONTROLLING FIRE WITHIN AN ENCLOSURE INVOLVING THE APPLICATION TO THE OBJECT SUBJECTED TO FIRE OF A COATING OF AN ABLATIVE FLUID TO FORM A LAYER OF SUFFICIENT THICKNESS SO THAT THE LAYER POSSESSES ABLATIVE PROPERTIES, SAID CAOTING BEING APPLIED FROM AN AUTOMATICALLY OPERABLE FIXED SOURCE IN A SUBSTANTIALLY CONTINUOUS STEAM IN SUFFICIENT QUANTITY TO MAINTAIN SAID LAYER AT SAID SUFFICIENT THICKNESS.

Description

P 1971 w. L. LIVINGSTON E A METHOD OF CONTROLLING FIRE 2 Sheets-Sheet 2 Filed Oct. 10. 1968 6532;: ms? nu 0m (Ni) *IVIHBLVW BAILV'ISV :IO HBAV I :10 SSBNMOIHL BALLV'IHINHO INVENTORS WILLIAM L. LIVINGSTON 8| RUSSELL W. PIERCE W A ORNEYS United States Patent Othce 3,605,900 Patented Sept. 20, 1971 3,605,900 METHOD OF CONTROLLING FIRE William L. Livingston, Sharon, and Russell W. Pierce, Hanover, Mass, assignors to Factory Mutual Research Corporation, Boston-Providence Turnpike, Mass.

Filed Oct. 10, 1968, Ser. No. 766,475 Int. Cl. A62c 1/00 U.S. Cl. 169--1A 12 Claims ABSTRACT OF THE DISCLOSURE A method of controlling fire within an enclosure involving the application to the object subjected to fire of a coating of an ablative fiuid to form a layer of sufficient thickness so that the layer possesses ablative properties, said coating being applied from an automatically operable fixed source in a substantially continuous stream in sufficient quantity to maintain said layer at said sufficient thickness.

BRIEF SUMMARY OF THE INVENTION A number of ways are known to extinguish a fire: (l) the flame can be cooled; (2) the various ingredients (reactants) passing to the flame, such as air, can be cooled; (3) the fuel array itself can be cooled; (4) the fire extinguishant (i.e., water) can be converted to steam which will then serve to inert the reactants passing through the flame and interfere with the fire producing process; and/or (5) the fire extinguishant (i.e., water) can be converted to steam which, in turn, will remove heat from the fire through the sensible heat of the steam.

Of the foregoing methods of extinguishing fires, the most effective approach to extinguishing the fire is the third, in which the fuel array itself is cooled, with the second technique being the next most preferable approach. Under presently employed fire extinguishing techniques employing water as the extinguishant, however, the water is employed to accomplish the first, fourth and fifth goals, with the second and third techniques rarely coming into play. In fact, present techniques of applying water as a fire extinguishant are such that little of the water employed is actually particularly effective in putting out the fire; on the contrary, much wastage is involved, resulting in extremely inefficient operation.

There are two key reasons as to why the use of water in conventional fire extinguishing techniques is inefficient. The first of these is that there is much bulk wastage of the water, with most of the water employed never actually reaching the fire or the immediate surroundings. This happens primarily because the water, when broken up into a spray, forms a wide distribution of droplets a good percentage of which becomes airborne and never has an opportunity to reach the fuel array. Secondly, even that water which reaches the fuel array is used ineifectively under current techniques. More specifically, because of the low viscosity of water, much of the water which reaches the fuel array runs quickly away from the fuel and is thus not available to fight the fire.

Notwithstanding the difliculties involved in the use of water for fire extinguishing purposes, water remains the most popular fire extinguishant for a number of reasons. In the first place, water has one of the highest heats of vaporization and specific heat of any of the materials which might be employed for this purpose. Secondly, while water convects heat, water has extremely low heat conductivity. Thirdly, with various exceptions, Water is a plentiful commodity available at low cost, making it particularly attractive from an economic standpoint.

The present invention makes possible the obtaining of all the advantages of water for fire extinguishing purposes but substantially eliminates the various disadvantages referred to above. Broadly stated, the present invention involves the controlling of fires within enclosures involving the application to the exposed surfaces of the objects subjected to such fire a coating of an ablative fluid material with sufiicient thickness so that the layer possesses ablative properties, such coating being applied from an automatically operable fixed source in a substantially continuous stream and in sufficient quantity to maintain said layer at said sutficient thickness.

It is accordingly a principal object of the present invention to provide a novel method of controlling fires within enclosures which makes possible the obtaining of the advantages resulting from the use of desirable fire extinguishants such as water while eliminating the disadvantages of such materials.

It is a further important object of the present invention to provide a novel method of controlling fires within enclosures comprising applying to the exposed surfaces of the objects subjected to such fire a coating of an ablative fluid material, said coating forming a layer of sufficient thickness so that said layer possesses ablative properties, said coating being applied from an automatically operable fixed source in a substantially continuous stream and in sufiicient quantity to maintain said layer at said sufiicient thickness.

It is still another important object of the present invention to provide a novel method of controlling a fire involving the use of an ablative fluid material, wherein said ablative fiuid material is formed from at least two components neither of which when separated from the other possesses ablative properties and wherein said components are mixed together to form said ablative material subsequent to the occurrence of said fire.

These and other important objects and advantages of the present invention will become more apparent in connection with the ensuing description, appended claims and the drawings accompanying the application.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph in which the ablation rate of the ablative material (both in terms of thickness reduction of the ablative material layer and in terms of the volume per unit time of ablative material which ablates) is plotted against the heat flux to which the combustible object is subjected;

FIG. 2 is a graph in which the rate of buildup of the thickness of ablative material is plotted against the sprinkler density of ablative material delivered from sprinkler heads to the combustible object; and

FIG. 3 is a graph in which the cumulative thickness of the layer of ablative material is plotted against time for different rates of application of the ablative material to the combustible object.

DETAILED DESCRIPTION OF THE INVENTION As previously noted, the present invention provides a method for controlling fires in enclosures through the use of an ablative material. An ablative material as comprehended within the present invention is a material which, when present in a layer of sufiicient thickness, will permit thermal energy to be transmitted through its exposed outer surface but not completely through said layer, said thermal energy being absorbed within said layer so as to immediately transform the material of said layer into vapor without internal convection of said material, said vapor leaving said layer through its outer surface. When used in this specification and claims, the terms ablative, ablation and the like shall be so construed.

Materials which will cause water to become ablative by virtue of increasing its thickness in the direction of the incident radiation and which are useful in the process of the present invention are readily available. One such material is a cross-linked ethylene maleic anhydride such as a material sold by the Monsanto Company under the designation EMA-91. Another such material which is useful is the diammonium-sodium salt of EMA-91, which is sold by the Monstanto Company under the designation EMA 94. The gels which are formed from such materials when they are mixed with water are extremely homogeneous (they are non-particulate in the sense that discrete particles are not visible to the naked eye), they can be easily pumped and sprayed in the form of desirable spray patterns with low pressure drops across the spray heads and they adhere well to vertical surfaces.

Other materials, such as a water-swellable, cross-linked polymer sold by the B. F. Goodrich Chemical Company under the designation Carbopol960, is also useful, as are materials such as those described in Katzer Pat. 3,354,084 and Bashaw et al. Pat. 3,229,769. All of these materials form gels when mixed with water in appropriate quantities and, when such gel is applied to a burning object in sufficient thickness, the gel possesses ablative properties as defined above.

The use of such materials as the foregoing to combat fires provides many advantages not obtainable through the use of conventional fire extinguishants such as water while retaining all the attributes of water as an extinguishant. Thus, because of the gel strength of the ablative material as compared with water, the ablative material once applied to the fuel array will not run off from the array as does water, eliminating much of the conventional wastage involved in present fire suppressant systems. In addition, when applied to the fuel array, the ablative materials are capable of being projected in the form of large droplets with practically no fines, such droplets being at least an order of magnitude (10 to 1) greater in diameter for the same flow rates and nozzle characteristics than water droplets. As a result, there is a minimum of airborne materials, thus eliminating the inefiiciency resulting from the use of water in which most of the water (viz., 90% or more for a 30-foot high fire plume) never has an opportunity to reach the fuel array. Furthermore, because such materials are comprised of approximately 99% water, they are characterized by the same low thermal conductivity and high heat of vaporization which characterize water. At the same time, the use of the ablative material produces large quantities of steam and, because a much larger percentage of such material reaches the burning fuel array than would have been reached by an equivalent quantity of water, the steam is better able to perform an inerting and heat removal function [items (4) and (5) on page 2, supra] than would the water.

Still further, because the ablative materials of the present invention are much more viscous than conventional suppressants such as water, water damage which ordinarily results from large fires is significantly reduced. More specifically, because of the high gellation of the ablative materials, such materials are not absorbed as readily by the combustibles and, therefore, water damage of such combustibles is significantly reduced.

The ablative materials of the present invention are particularly effective against large, energetic fires whereas currently employed water suppressant systems are not. In deed, the latter becomes less effective as the fire increases on a per square foot basis. The reason for this is simple. For a water suppressant system to make available the necessary quantity of water to establish a suitable spray (taking into account the low etficiency of water suppressant systems) for the majority of sprinklers in a system (which would be necessary in the case of a large fire), the cost of the system would be prohibitive. The result is that conventional water systems are designed to cope only with less than huge fires so that the water pressure at each sprinkler head will provide moderate effectiveness in combating such a fire. Fire suppressant systems employing ablative materials in the present invention, on the other hand, are uniquely capable of coping with large fires at reasonable cost since they require much lower flow rates per head due to their increased efficiency. At the same time, the systems of the present invention reduce the total cost of the fire protection for a given enclosure (due to the permissible use of smaller diameter pipes, pumps, etc.) while permitting the use of more suitable materials.

While, as noted from the Katzer and Bashaw et al. patents cited previously, gelling agents such as those noted above have been suggested for use in controlling the spread of fire, the teachings of these and similar references are deficient in a number of significant respects. More specifically, all of these teachings contemplate the employment of the gel material in fire fighting situations other than those employed in fixed systems involving automatically operable fixed sources of extinguishant of the type used within buildings and other enclosures such, for example, as those involving the use of fixed sprinkler heads and the like. Indeed, not only do the teachings of such references contemplate the use of such gelling agents in connection with outdoor fire fighting of the municipal type but, more significantly, there are a great number of reasons as to why those skilled in the art would have considered inappropriate the use of such materials in connection with an automatically operable fixed source within an enclosure.

By way of example, automatically operable fixed fire extinguishing systems invariably are associated with extensive fixed piping networks employed to deliver the extinguishant to the source of the fire at the appropriate time. Because gels such as those described in the Katzer and Bashaw et al. patents are not only difficult to pump but would pick up debris, scale and the like from conventional steel pipelines so as to create plugging problems, such gel materials would have seemed ill-suited to the demands of the conventional automatically operable fixed source system within a building or other enclosure. Furthermore, the shelf life of these gel materials once mixed with water is short. Since most sprinkler systems never become operative, storage of the extinguishant material is a critical problem and the short shelf life of the materials would obviously be likely to compound rather than to meet this problem.

In accordance with the present invention, however, it has been found that the foregoing problems can be readily eliminated in connection with the use of fire suppressant systems employing ablative materials. More specifically, the substantially increased etficiency and decreased costs resulting from the use of ablative materials as previously noted makes it feasible to use systems having pipes of smaller diameter and lower wall thickness while obtaining at least the equivalent, if not improved, results. The necessary consequence of this fact is that it becomes commercially feasible to use hardware made from more suitable materials (such as stainless steel, copper or the like) which are of increased corrosion resistance as compared with conventional iron pipe and the like and which therefore do not present the plugging problems which would be presented in conventional steel pipelines.

In addition to the foregoing, it is contemplated in the practice of the present invention to avoid the shelf-life problem created by the use of material such as is described in the Katzer and Bashaw et al. patents by deferring the mixture of the gelling agent with the fluid carrier (i.e., water) to a point subsequent to the occurrence of the fire. Because the shelf-life problem does not exist until the gelling agent is mixed with the water, the process of the present invention essentially eliminates this problem.

Furthermore, the ablative nature of the materials disclosed in the Katzer and Bashaw et al. patents was not recognized by the prior art. As previously noted, an ablative material is one which, when present in a layer of sufficient thickness, will permit thermal energy to be transmitted through its exposed outer surface but not completely through said layer. On the contrary, an ablative material absorbs such thermal energy within its body so as to immediately transform it into vapor without internal convection of said material, such vapor leaving the material through its outer surface. The significance of this phenomenon in connection with fire fighting is great. More specifically, ablative materials will pick up the heat emanating from a fire radiatively (the primary mechanism of the fire growth is radiation) in its outer layers. If the ablative material is added to the fuel array in a manner such as to build up and maintain a minimum thickness of such material, the heat energy from the fire (which is mostly infrared) will be transmitted through the exposed outer surface of the ablative material but will not pass through it to the fuel array. The absorbed heat energy rapidly transforms such ablative material in the area of absorption into vapor without internal convection of the ablative material, with the vapor leaving the layer of ablative material through its outer surface as steam. Since the heat energy is not transmitted completely through the layer of ablative material under such circumstances, the temperature drop across the entire layer of ablative material will never exceed 212 F. and the water, being a reasonably good thermal insulator and constituting the bulk of the ablative material, will work to extinguish the fire.

As the material ablates in the course of the fire, the ablative material remaining behind increases in viscosity. The result is that ablative material is left clinging to the burning fuel array with a continued capability of fire fighting and, so long as the thickness of the layer of ablative material on the fuel array is above the minimum at which such material will ablate, the material will serve to protect the exposure.

As noted, in order to obtain the advantages of the ablative materials described above, a sufficient quantity of ablative material must be added to the fuel array to maintain a minimum thickness layer of ablative material so that the ablative layer will absorb thermal radiation from the fire and will prevent infrared radiation from passing through to the fuel array. This minimum thickness is about 0.04 inch and is preferably as thick as possible. To accomplish this goal effectively, it is necessary to apply the ablative material in a substantially continuous stream to those sites requiring fire suppressant so as to continu ously replenish material which hasablated and to maintain such minimum thickness of the layer. As a practical matter, this will not be obtainable from a portable system of the nature used by municipal fire fighters since, in such situations, the flow of suppressant to a given spot is not continuous but staggered as the stream of suppressant is moved across the extremities of the burning fuel array.

Given the minimum thickness of ablative material which must be present on a combustible surface to prevent that surface from burning, the control of a fire suppressant system to obtain the desired results may be readily determined by means of charts such as those illustrated in FIGS. 1-3. As noted, FIG. 1 is a graph in which the ablation rate of the ablative material (both in terms of thickness reduction of the ablative material layer and in terms of the volume per unit time of ablative material which ablates) is plotted against the heat flux to which the combustible object is subjected. Most fires are in the 0-10 (B.t.u./hr./ft. heat flux range with the heat flux of a particular fire being readily determinable by those skilled in the art. (The heat flux rate set forth in FIG. 1 represents black body radiation heat fluxes.) From the graph of FIG. 1, once the heat flux for a given type of fire is determined, the ablation rate of the ablative material may be readily calculated either in terms of thickness reduction in inches per minute or in terms of loss of ablative material expressed in terms of gal./min./ ft. Since the possession by the ablative material of ablative properties depends upon the maintaining of the material in a layer of minimum thickness, the desired minimum thickness may be maintained (once having determined the ablation rate from FIG. 1) by referring to the graph of FIG. 2 in which the rate of thickness increase is plotted against the sprinkler density of ablative material delivered from the sprinkler heads to the combustible object.

By way of example, if it is determined that a given fire involves a heat flux of 10, from FIG. 1 it can be readily noted that the ablation rate of the ablative material is about .0l5 in./min. From the chart of FIG. 2, it can be readily determined that in order to replace an ablative material loss of .015 in./min., a sprinkler density of approximately .009 gal./min./ft. must be employed.

The graph of FIG. 3 is also useful in the foregoing connection in that this graph provides information as to the amount of time required to build up a given thickness of ablative material at different sprinkler flow rates. As will be apparent, the information on this graph will be helpful in determining the conditions which must be observed at the outset of a fire situation to build up the minimum thickness of ablative material.

It should be noted in connection with the foregoing that because the ablative materials of the present invention do not exhibit their ablative properties until a minimum thickness of such material is applied to the fuel array, there will be a time lag when the ablative material is first applied to the burning fuel array until the ablative material is capable of performing its fire suppressant function. In light of this fact, it is within the contemplation of the present invention toemploy an initial ablative material flow rate when the fire suppressant system is first rendered operative which is higher than that ultimately required to maintain the minimum thickness of ablative material. The flow rates to be employed for such purpose may be readily determined from the charts of FIGS. 1-3.

(Note: While it is true that ablation rates will vary depending upon the type of ablative material employed, variations between ablative materials are sufficiently small so that the data set forth in the charts of FIGS. 1-3 are generally applicable.)

As will be apparent to those skilled in the art, graphs such as those of 'FIGS. 1-3 will be used to determine the desired characteristics of the installation required for a given hazard. Once the system has been installed, the initial and subsequent flow rates will be fixed and will not be varied in magnitude.

As previously noted, most of the difficulties involved in the use of the ablative materials of the present invention for controlling fires from an automatically operable fixed source within an enclosure are obviated by deferring the mixture of the gelling agent with the appropriate fluid carrier (e.-g., water) to a point subsequent to the occurrence of a fire. Because the residence time of the ablative fluid material in the piping system will be rather short under such circumstances, it is necessary in the practice of the present invention to take steps to assure that the fluid material which is directed to the burning fuel array has had suificient opportunity to develop the required ablative properties. Toward this end, it is desirable to employ a gelling agent in finely divided powder form (viz., it should be 200 mesh) to facilitate mixing with the fluid carrier. Such mixing can be further facilitated by first slurrying the gelling agent With a water-miscible solvent which will not hydrolyze and which is non-reactive with the gelling agent (such as methanol, ethanol, acetone or the like), which slurry is then injected into admixture with the fluid carrier. An appropriate slurry is one containing approximately 25-75% by weight of the solid gelling agent, with approximately 50% by weight of solid being preferred.

Generally speaking, the amount of gelling agent (on a solids basis) which should be mixed with water to form the desired gel should be about 0.1-0.5 and preferably about 0.2% by weight. The upper limit is a practical one, since the use of a gelling agent above 0.5% by weight is not only too expensive but, with at least certain of the gelling agents employable in this process, produces a gel which is too viscous for practical use for fire control. Below the lower preferred limit of 0.1%, the gel may not be viscous or gelled enough and the amount of cling (capability of sticking to vertical surfaces) is too low to permit the ablative material to build up on vertical surfaces in sufficient thickness to be ablative.

In a previous discussion it was noted that the preferred fluid for formulating the ablative materials of the present invention is water. It is contemplated in connection with the practice of the present invention, however, that materials other than water can be employed, provided they permit the formation of a material having gel strength following the addition of an appropriate gelling agent and;

(1) Will cling to vertical surfaces in a layer thicker than will transmit infrared while absorbing such infrared within its confines;

('2) Will not permit substantial convection;

(3) Has low thermal conductivity;

(4) Has a high heat of vaporization;

(5) Has a boiling point which is below the ignition temperature of the fuel to which it is applied (wood ignites at about 400 C.);

(6) Is inexpensive; and

(7) Has a good shelf life at least before mixing with the gelling agent.

In order to demonstrate the eifectiveness of the process of the present invention, a test was conducted as follows: 25 wooden pallets having dimensions of 5 /2 x 4 x 4', each weighing approximately 100 pounds and having a moisture content of about 4% by weight, were arranged in a pile approximately 12' high, the total weight of the pile being about 2300 lb. These pallets were ignited by directing eight jets of burning Esso regular gasoline uniformly about the base of the pile at a total gasoline dis charge rate (for all eight jets) of 1 /2 gals/min. After the jets were open for approximately six minutes, an ablative fluid spray nozzle centrally positioned above the top of the pile of pallets was opened to apply ablative fluid over a 200 sq. ft. area in a square pattern onto the blazing pile of pallets.

The ablative fluid employed was made by mixing with water a slurry containing EMA94 (a diammoniumsodium salt of a cross-linked ethylene maleic anhydride made by the Monsanto Company), isopropanol and Cab- O-Sil (a colloidal pyrogenic silica pigment manufactured by the Cabot Corp.). The slurry was formed by mixing approximately 50 parts by weight of isopropanol with 15 parts by weight of Cab-O-Sil (the latter material forms a gel and removes water from the isopropanol) to form a gel, following which approximately 35 parts by weight of 200 mesh EMA-94 powder was added to the gel with mixing. The slurry thus formed was injected from /2 diameter copper tubing into the center of a water stream flowing in 3" diameter steel pipe by means of a Moyno positive displacement slurry pump so that the density of ablative material directed onto the blazing pile of pallets was approximately 0.2 gal./min./ft. The amount of slurry added to the water was about 1% by weight and, since the EMA-94 represented about 35% by weight of the slurry, the amount of gelling agent actually added to the water was about .35 by weight. The square spray pattern was obtained by using a solid pattern square spray nozzle manufactured by the Spray System Co. under N0. 2H290SQ.

Though a considerable portion of the pile of pallets was consumed by fire before the ablative material was applied to the burning pile, the ablative material was able to reach the pallets through the fire plume (which, at the time the ablative material was first turned on, was approximately feet above the top of the pile), accumulate and seep down through the pile.

The normal burning rate for wood pallets of the type used in the foregoing test reaches approximately 80 lb. of wood per minute and, using water as the extingunishant,

the pile of wood used in this test would burn to ashes at water densities of at least 0.6 gal./min./ft. In the foregoing test, on the other hand, the burning rate of the pallets, which had reached approximately 50 lb. per minute at the time the ablative material was first applied to the pile (at a rate of only 0.2 gal./rnin./ft. had been reduced to about 5 lb. per minute Within 23 minutes and remained at that level for approximately one hour at which point the fire was completely extinguished and a substantial portion of the pallets remained unburned. It is thus clear that the use of the process of the present invention resulted in a significant improvement over the use of a conventional water extinguishant.

The invention may be embodied in other specific forms without departing from the spirit or essential characteris tics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come Within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

What is claimed is:

1. A method of controlling fire in a space to be protected comprising supporting at an elevated position within said space an automatically operable fixed fire extinguishing system having a plurality of spaced extinguishant dispersing heads connected to a network of pipes for supplying ablative fluid material to said heads, activating in response to a fire in said space at least one of said heads to produce a spray of droplets of ablative fluid material to apply to the exposed surfaces of an object in said space beneath said head subjected to said fire a coating of said ablative fluid material, said coating forming a layer of sufficient thickness so that said layer possesses ablative properties to permit thermal energy to be transmitted through its exposed outer surface but not completely through said layer, said thermal energy being absorbed within said layer so as to transform the material of said layer into vapor without internal convection of said material, said vapor leaving said layer through its outer surface, said spray of droplets being applied continuously and in sufficient quantity to maintain said layer at said sufl'lcient thickness.

2. A method as defined in claim 1 wherein said ablative fluid material is formed from at least two components, neither of said two components when separated from the other possessing ablative properties, and wherein sa1d components are mixed together to form said ablative material subsequent to the occurrence of said fire.

3. A method as defined in claim 1 wherein the major component of said ablative fluid material is water.

4. A method as defined in claim 3 wherein said fluid material is comprised of water containing a component which will cause said fluid material to possess ablative properties when said material is present in a layer of suflicient thickness.

5. A method as defined in claim 4 wherein said component is a cross-linked ethylene maleic anhydride, said fluid material containing about 0.10.5% by weight of said component.

6. A method as defined in claim 1 wherein said thickness is at least about 0.04 inch.

7. A method as defined in claim 5 wherein said fluid material contains about 0.2% by weight of said component.

8. A method as defined in claim 5 wherein said fluid material contains about 0.1-0.2% by weight of said component.

9. A method as defined in claim 5 wherein said component is contained in a slurry having about 2575% by weight of said component, and wherein said slurry is mixed together with said water to form said ablative fluid material subsequent to the occurrence of said fire.

10. A method as defined in claim 9 wherein said slurry contains about 50% by weight of said component.

11. A method as defined in claim 1 wherein said spray of droplets is applied initially at a high rate to build up and maintain said layer of suflicient thickness and subsequently at a reduced rate sufficient to maintain said layer of sufiicient thickness.

12. A method as defined in claim 1 wherein each of said heads is a solid pattern direct spray nozzle, and wherein the sizes of said droplets are several times larger than droplets of water from the same nozzles at the same pressures and flow rates.

References Cited UNITED STATES PATENTS 2,696,266 12/1954 Tuve 169-14 3,354,084 11/1967 Katzer 16914UX 10 Savins l69l Dale l69lX Bashaw et a1. l69lX Urquhart 169-15 Blakeslee et al 222-70 Valente 169-5X Bashaw et al 169--l Nicholls et a1. 26078.5

U.S. Cl. X.R.

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